In the development of radio communication systems, such as mobile communication systems (like for example GSM (Global System for Mobile Communication), GPRS (General Packet Radio Service), UMTS (Universal Mobile Telecommunication System) or the like), efforts are made for an evolution of the radio access part thereof. In this regard, the evolution of radio access networks (like for example the GSM EDGE radio access network (GERAN) and the Universal Terrestrial Radio Access Network (UTRAN) or the like) is currently addressed. Such improved radio access networks are sometimes denoted as evolved radio access networks (like for example the Evolved Universal Terrestrial Radio Access Network (E-UTRAN)) or as being part of a long-term evolution (LTE) or LTE-Advanced. Although such denominations primarily stem from 3GPP (Third Generation Partnership Project) terminology, the usage thereof hereinafter does not limit the respective description to 3GPP technology, but generally refers to any kind of radio access evolution irrespective of the underlying system architecture. Another example for an applicable broadband access system may for example be IEEE 802.16 also known as WiMAX (Worldwide Interoperability for Microwave Access).
In the following, for the sake of intelligibility, LTE (Long-Term Evolution according to 3GPP terminology) or LTE-Advanced is taken as a non-limiting example for a broadband radio access network being applicable in the context of the present invention and its embodiments. However, it is to be noted that any kind of radio access network may likewise be applicable, as long as it exhibits comparable features and characteristics as described hereinafter.
In the development of cellular systems in general, and access networks in particular, relaying has been proposed as one concept. In relaying, a terminal or user equipment (UE) is not directly connected with an access node such as a radio base station (e.g. denoted as eNodeB or eNB) of a radio access network (RAN), but via a relay node (RN) which is connected to the access node. Relaying by way of relay nodes has been proposed as a concept for coverage extension in cellular systems. Apart from this main goal of coverage extension, introducing relay concepts can also help in providing high-bit-rate coverage in high shadowing environments, reducing the average radio-transmission power at the a user equipment (thereby leading to long battery life), enhancing cell capacity and effective throughput, (e.g. increasing cell-edge capacity and balancing cell load), and enhancing overall performance and deployment cost of radio access networks.
FIG. 1 shows a schematic diagram of a typical deployment scenario of a relay-enhanced cellular system, such as e.g. a LTE or LTE-Advanced RAN with radio-relayed extensions. As shown in FIG. 1, UEs at disadvantaged positions such as a cell edge and/or high shadowing areas are connected to a so-called donor base station (DeNB) via a respective relay node RN. Generally, any one of the relay nodes may be stationary/fixed or mobile.
The coverage or service area of a relay node may be referred to as relay cell, and the coverage or service area of a donor base station may be referred to as donor cell. Accordingly, both the DeNB as well as the RNs may be regarded as access nodes or base stations of an access network, possibly as access nodes or base stations of different hierarchical level in terms of logical and/or structural network deployment.
FIG. 2 shows a schematic diagram of an interface definition of a relay-enhanced cellular system. As shown in FIG. 2, the (wireless) link between donor base station (DeNB) and relay node (RN) may be referred to as Un link or relay link, and the (wireless) link between the relay node (RN) and the terminal or user equipment (UE) may be referred to as Uu link or access link.
In the context of LTE and LTE-Advanced, a Layer 3 (L3) RN, also referred to as Type I RN, is currently taken as a baseline case for the study on relay extensions. Such a relay node is exemplarily assumed for the further description. The Type I relay node appears as a normal base station (eNB) towards its served terminals or user equipments (UE), and appears as a terminal or user equipment towards it serving donor base station (DeNB). The Type I relay node performs proxy functionality as to relay traffic and signaling between UE and DeNB.
In the development of cellular systems in general, and access networks in particular, the concept of closed subscriber groups (CSG) has been proposed. A cell with a closed subscriber group (CSG), also referred to as CSG cell is only allowed to be accessed by a terminal or user equipment when this terminal or user equipment is a member of the CSG of that cell or, stated in other words, is a member of that cell. In this regard, the parameters csg-indication and csg-identity are defined as CSG-related parameters for handling and managing access of CSG cells. The parameter csg-indication indicates whether or not a cell is a CSG cell, and the parameter csg-identity defines the identity of the CSG within the cellular system the cell belongs to. When csg-indication is set to TRUE for a specific cell, the terminal or user equipment is only allowed to access this cell, if the csg-identity matches an entry in the CSG whitelist being stored in the terminal or user equipment.
The concept of closed subscriber groups is also applicable to relay-enhanced cellular systems. In such case, any relay cell may be a CSG cell or not, and any donor cell may be a CSG cell or not. The CSG-related parameters of the individual cells may be transferred by being included in System Information Block 1 (SIB1) according to current specifications so as to be advertised between RN and DeNB.
In a relay-enhanced cellular system supporting the CSG concept, a two-fold user access control procedure is specified to be conducted. That is, a CSG membership may be checked at the RAN (radio access network) side, and (if successful) a CSG membership may be validated at the CN (core network, e.g. evolved packet core (EPC)) side.
A user (i.e. its UE) trying to get access to a cellular system via a relay cell, has to be member of the CSG relay cell (if the relay cell is a CSG cell) while the donor cell is invisible to the user.
No problems arise in this regard when, as conventionally, a relay cell is always configured with the same CSG settings as its serving donor cell. However, such restriction is undesirable and/or inappropriate in view of current demands e.g. in terms of flexibility of deployment and usage.
When a relay cell may be configured with different CSG settings as its serving donor cell (which may easily be the case e.g. when the corresponding system information is configured independently, e.g. by RN OAM for the relay cell and by DeNB OAM for the donor cell), problems may arise for the CSG membership check at the RAN side, if the DeNB represents a CSG cell while the RN does not represent a CSG cell, and/or if both DeNB and RN represent CSG cells but the RN has a different CSG identity as compared with its DeNB.
FIG. 3 shows a schematic diagram of an exemplary deployment scenario of a relay-enhanced cellular system where the DeNB represents a CSG cell while the RN does not represent a CSG cell.
As shown in FIG. 3, the DeNB represents a CSG cell where the csg-indication is set to true and the csg-identity is advertised to its coverage including the RN. The RN is an open cell with csg-indication set to false. If the csg-identity is not in the UE's whitelist, i.e. the UE is not a member of the DeNB or its cell, the UE in position A is not allowed to access to the DeNB. However, if the UE moves to position B which locates in the coverage of the RN, the UE is able to access the open cell of the RN and, therefore, to use back-haul resources between the DeNB and the RN. That is, the existence of the RN and its open cell allows the invasion of non-member UEs to the DeNB and the consumption of the radio resource of the DeNB, which violates the proprietary feature of CSG cells.
FIG. 4 shows a schematic diagram of an exemplary deployment scenario of a relay-enhanced cellular system where both DeNB and RN represent CSG cells but the RN has a different CSG identity as compared with its DeNB.
As shown in FIG. 4, both the DeNB and the RN represent CSG cells but with different CSG identities. A conceivable scenario of such deployment situation may for example be in an office environment, where the DeNB is deployed to provide coverage to the whole building, while the RN is implemented in each floor to serve the staff in the floor only. If the UE (i.e. an entry in its CSG whitelist) matches only the csg-identity of the RN (but not the csg-identity of the DeNB), the UE could access to the RN and then, via the RN, to the DeNB indirectly. That is, the existence of the RN and its differently set CSG cell allows the invasion of non-member UEs to the DeNB and the consumption of the radio resource of the DeNB, which violates the proprietary feature of CSG cells.
In view thereof, mechanisms are needed for a correct membership handling of CSG cells in relay-enhanced cellular systems at the RAN side.
When both relay cell and a donor cell may be configured as CSG cells, further problems (besides the above problems regarding CSG membership check at the RAN side), may arise for the CSG membership validation at the CN side.
Conventionally, after a connection is established between DeNB and RN and between RN and UE, the UE sends an Attach Request message. The RN then includes the Attach Request message together with, among others, its csg-identity (hereinafter denoted as CSG ID) into the initial UE message and then sends this initial UE message to the DeNB (e.g. as in the S1AP protocol). According to an agreed architecture for relay-enhanced cellular systems in 3GPP, the DeNB performs the proxy functionality. Thus, the same message with the RN CSG ID is forwarded to the mobility management entity (MME) at the core network such as the evolved packet core (EPC), or the DeNB replaces the RN CSG ID with its own CSG ID, i.e. the DeNB CSG ID, and forwards it to the MME at the core network. Since the RN is invisible to the MME according to current relay architectures, the MME may only consider the single CSG ID included in this initial UE message as CSG ID of the DeNB, irrespective of whether it is the DeNB CSG ID or the RN CSG ID. Upon receiving this message with the single CSG ID, the MME validates the UE's membership according to the CSG ID included. That is, whatever the DeNB proxies (RN CSG ID or DeNB CSG ID), the MME only sees one CSG ID and may, thus, handle (the validation of) the CSG membership of the UE only with this single CSG ID. That is, the coexistence of the RN CSG cell and the DeNB CSG cell impedes a proper handling of CSG memberships for all deployment levels of a relay-enhanced cellular system, which violates the proprietary feature of CSG cells.
In view thereof, mechanisms are needed for a correct membership handling of CSG cells in relay-enhanced cellular systems at the CN side.
Thus, there do not exist any feasible mechanisms for properly and correctly handling closed subscriber groups in a relay-enhanced system, such as for example in relay-enhanced access networks.
Accordingly, there is a demand for mechanisms for properly and correctly handling closed subscriber groups in a relay-enhanced system.